The compounds of the invention are known in the art. N-Isopropylheteroaryloxyacetanilides are described in European Patent Publication Number 0348737, published on Jan. 3, 1990, which in particular discloses N-isopropyl-(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)-(4'-fluoro-oxyacetan itide) as compound 34. 4-Benzoylisoxazoles are described in for example European Patent Publication Numbers 0418175, 0487357, 0527036 and 0560482. In particular, European Patent Publication Number 0560482, published on Feb. 10, 1993, discloses 5-cyclopropyl-4-(2-methylsulphonyl-4-trifluoromethylbenzoyl)isoxazole. Each of these publications teach that these compounds possess herbicidal activity. However, neither of the publications teach specific anti-gramineous properties in the compounds. EP-A1-0348737 teaches that the acetanilide compounds may be applied at from 0.01 to 10 kg of active compound per hectare of soil surface, preferably 0.05 to 5 kg per ha; no details of the precise dose rates used for the application of the compounds to treat graminea EP-A1-0527036 teaches that the compounds of the invention possess high activity in comparison with known compounds against certain weed species.
Hence the literature does not teach mixtures of these compounds; nor does it suggest that such a mixture would be expected to be particularly usefull as an anti-graminicide. At present the most commonly used mixture in this area of weed science comprises metolachlor and atrazine. These are frequently recommended for use at dose rates from about 1 to 2.5 kg/ha of metolachlor and from 0.5 to 2.5 kg/ha of atrazine. An object of the invention is thus to provide a mixture suitable as a graminicide which may be used at reduced dose rates of active compound in comparison with the known products.
Remarkably, it has been found that the mixture of N-isopropyl-(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)-4-(4'-fluoro-oxyacet anihide) with 4-benzoylisoxazole derivatives provides effective control of an extremely wide range of both monocotyledon and dicotyledon weeds at reduced dose rates when compared with these known compounds. Moreover it provides effective control together with selectivity in important crop species such as maize.
Surprisingly, and in addition to this, it has been found that in certain conditions the combined herbicidal activity of certain 4-benzoylisoxazoles with N-isopropyl-(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)-4-(4'-fluoro-oxyacet aanilde) for the control of certain weed species e.g. Setaria viridis, Echinochloa crus-galli, Amaranthius retroflexus and Polygonum lapathifolium, is greater than expected, without an unacceptable increase in crop phytotoxicity, i.e the mixture shows synergism as defined by Limpel, L. E., P. H. Schuldt and D. Lamont, 1962, 1. Proc. NEWCC 16, 48-53, also known as the Colby formula (Colby S. R., 1967, Weeds 15, 20-22), using the formula: ##EQU1## where E=the expected percent inhibition of growth by a mixture of two herbicides A and B at defined doses.
X=the percent inhibition of growth by herbicide A at a defined dose. PA1 Y=the percent inhibition of growth by herbicide B at a defined dose. PA1 (a) a 4-benzoylisoxazole of formula (I): ##STR2## wherein R is hydrogen or --CO.sub.2 R.sup.3 ; PA1 (b) N-isopropyl-(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)-4-(4'-fluoro-oxyacet anilide) having the formula (II): ##STR3## PA1 (a) a 4-benzoylisoxazole derivative of formula I as defined above; and PA1 (b) N-isopropyl-(5-trifluoromethyl-1,3,4-thiadiazol-2-yl)-4-(4'-fluoro-oxyacet anilide); PA1 in association with, and preferably homogeneously dispersed in, a herbicidally acceptable diluent or carrier and/or surface active agent. PA1 (a) 4-benzoylisozaxole of formula I as defined above; and PA1 (b) N-isopropyl-(5-trifiuoromethyl-1,3,4-thiadiazol-2-yl)-4-(4'-fluoro-oxyacet anilide), PA1 as a combined preparation for simultaneous, separate or sequential use in controlling the growth of weeds at a locus.
When the observed percentage of inhibition by the mixture is greater than the expected value E using the formula above the combination is synergistic.
This remarkable synergistic effect gives improved reliability in controlling these competitive weeds of many crop species, and contributes to a considerable reduction in the amount of active ingredient required for weed control.